magus of the unseen gain control of target artifact magic ability stack faq

[vacation]

If I tap and use ability of my Serrated Biskelion

In response, other player gains control of it with Magus of the Unseen

who decides where the -1/-1 counter from the ability goes to? Because I activated the ability, but when it is resolving the artifact is under his control?

[Arix]

You choose targets for the ability as you activate it, not as it resolves.